Submicron-scale and lower-micron graphitic fibrils as an anode active material for a lithium ion battery

ABSTRACT

The present invention provides a lithium ion battery anode material comprising a submicron-scaled graphitic fibril having a diameter or thickness less than 1 μm but greater than 100 nm, wherein the fibril is obtained by splitting a micron-scaled carbon fiber or graphite fiber along the fiber axis direction. This type of graphitic fibril exhibits exceptionally high electrical conductivity, thermal conductivity, elastic modulus, and strength. The anode material exhibits a high reversible capacity and good charge/discharge cycling stability for both low and high charge rate conditions. Another preferred embodiment of the present invention is an anode active material containing a graphitic fibril with a diameter greater than 1 μm but less than 6 μm obtained by splitting a carbon fiber or graphite fiber of at least 6 μm in diameter.

This application is a continuation of U.S. patent application Ser. No.12/592,970 (Dec. 7, 2009)—A. Zhamu and B. Z. Jang, “SUBMICRON-SCALEGRAPHITIC FIBRILS, METHODS FOR PRODUCING SAME AND COMPOSITIONSCONTAINING SAME”. The entire contents of each of which are herebyincorporated by reference. This instant application claims the benefitsof U.S. application Ser. No. 12/592,970.

FIELD OF THE INVENTION

The present invention relates to a new type of carbon fibers or graphitefibers that are referred to as sub-micron graphitic fibrils(characterized by having a diameter or thickness lower than one micronbut greater than 100 nm) or lower-micron graphitic fibrils (having adiameter equal to or greater than 1 μm, but less than 6 μm), which areproduced by intercalating, exfoliating, and separating micron-sizedcarbon fibers or graphite fibers (greater than 6 μm in diameter andmostly approximately 12 μm in diameter).

BACKGROUND

Carbon is known to have four unique crystalline structures, includingdiamond, graphite, fullerene, and carbon nano-tubes (and itslarger-diameter cousins—carbon nano-fibers or CNFs). The carbonnano-tube (CNT) refers to a tubular structure grown with a single wallor multi-wall, which can be conceptually obtained by rolling up agraphene sheet or several graphene sheets to form a concentric hollowstructure. A graphene sheet or basal plane is composed of carbon atomsoccupying a two-dimensional hexagonal lattice. Carbon nano-tubes have adiameter on the order of a few nanometers to a hundred nanometers.Carbon nano-tubes can function as either a conductor or a semiconductor,depending on the rolled shape and the diameter of the tubes. Itslongitudinal, hollow structure imparts unique mechanical, electrical andchemical properties to the material. Carbon nano-tubes are believed tohave great potential for use in field emission devices, hydrogen fuelstorage, rechargeable battery electrodes, and as compositereinforcements.

However, CNTs are extremely expensive due to the low yield and lowproduction rates commonly associated with all of the current CNTpreparation processes, such as arc discharge, laser ablation, chemicalvapor deposition (CVD), and catalytic CVD (CCVD). The high materialcosts have significantly hindered the widespread application of CNTs.Earlier CNT production methods include those disclosed in the followingpatents: H. G. Tennent, “Carbon Fibrils, Method for Producing Same andCompositions Containing Same,” U.S. Pat. No. 4,663,230 (May 5, 1987); C.Snyder, “Carbon Fibrils,” U.S. Pat. No. 5,707,916 (Jan. 13, 1998).

Carbon nano-fibers (CNFs) are prepared from CVD, CCVD, orelectro-spinning of polymer nano-fibers followed by carbonization.Electro-spinning has not been regarded as a mass-production method dueto the limited amount of material that can be electro-spun with onehollow needle head. An example of the process to produce polymernano-fibers via electro-spinning is given in D. H. Reneker, et al,“Processes for Producing Fibers and Their Use,” US Pub. No. 2009/0039565(Feb. 12, 2009).

The CNFs produced by the CVD and CCVD processes are commonly referred toas vapor-grown carbon nano-fibers (VG-CNFs). VG-CNFs have beenextensively investigated in recent years and are commercially availableat very high prices (e.g., $300/Kg). The following are some examples ofCNF production processes: D. J. C. Yates, et al., “Production of CarbonFilaments,” U.S. Pat. No. 4,565,683 (Jan. 21, 1986); S. H. Yoon,“Ultra-fine Fibrous Carbon and Preparation Method Thereof,” US Pub. No.2009/0075077 (Mar. 19, 2009); S. H. Yoon, “Ultra-fine Fibrous Carbon andPreparation Method Thereof,” U.S. Pat. No. 7,470,418 (Dec. 30, 2008); S.H. Yoon, “Porous Filamentous Nano Carbon and Method of Forming theSame,” US Pub. No. 2009/0004095 (Jan. 1, 2009); G. Oriji, “Carbon NanoFiber, Production and Use,” US Pub. No. 2009/0008611 (Jan. 8, 2009); J.L. Gonzales Moral, et al., “Carbon Nanofibers and Procedure forObtaining Said Nanofibers,” US Pub. No. 2009/0035569 (Feb. 5, 2009).

VG-CNFs and related CNTs have several drawbacks that have significantlyconstrained their scope of application:

-   -   (a) Both CVD and CCVD processes typically involve using a        catalyst and the catalyst particles (e.g., transition metal nano        particles or their alloys) usually become part of the resulting        CNF or CNT structure. Normally, there is a significant amount of        catalyst used in these processes. The residual catalyst, even        just a trace amount, is considered undesirable in many        applications. For instance, Fe is viewed as detrimental to the        performance of a lithium ion battery if CNFs or CNTs are used as        an anode active material. Catalytic particles can also catalyze        or accelerate thermal or chemical degradation of a polymer        matrix composite material.    -   (b) The CVD or CCVD processes intrinsically introduce a        significant amount of impurities into the resulting CNFs or        CNTs. It is not unusual to find a purity level (graphitic carbon        content) in a CNF less than 80-90%.    -   (c) Depending upon the processing conditions, the graphene        planes in different CNFs may be oriented at different angles        with respect to the fiber axis. Furthermore, the graphene planes        may be curved as a cup-shape or a cone-helix structure, which        are not conducive to achieving high strength or modulus along        the fiber axis. In one example, the fibers consist primarily of        conical nano-fibers, but can contain a significant amount of        bamboo nano-fibers. Most conical nano-fibers consist of an        ordered inner layer and a disordered outer layer. When subjected        to a thermal treatment above 1,500° C., some CNFs can undergo a        structural transformation with the ordered inner layers changing        from a cone-helix structure to a highly ordered multiwall        stacked cone structure. The bamboo nano-fibers can have a        tapered multiwall nanotube structure for the wall and a        multi-shell fullerene structure for the cap of each segment,        surrounded by a disordered outer layer. When these fibers are        heat treated, the disordered outer layers transform to an        ordered multiwall nanotube structure and merge with the wall of        each segment. The end caps of each segment transform from a        smooth multiwall fullerene structure to one consisting of        disjointed graphene planes. Such a thermally induced instability        in the CNF structure is an undesirable feature of CNFs for        high-temperature applications (e.g., as a reinforcement in a        carbon matrix composite).    -   (d) The CNFs typically have a continuous thermal carbon        overcoat, which is a result of the thermal decomposition effect        during the CNF formation process via the CVD, CCVD, or        carbonization of electro-spun polymer nano-fibrils. Although        this carbon overcoat could serve as a protective layer for the        internal graphitic crystallites in some applications, the        overcoat is detrimental to many other engineering applications.        For instance, this overcoat makes it difficult to chemically        functionalize the CNF surface, thereby inhibiting the formation        of a strong bond between a CNF and a polymer matrix in a polymer        composite. This hard overcoat also makes it difficult for        lithium ions to enter or leave the CNF if the CNF is used as a        lithium ion battery anode material. In a similar manner, a CNT        has a complete, continuous graphene plane wrapped around the        tube axis, which has few active sites where chemical        functionalization can occur. Hence, chemical functionalization        occurs only at the edge unless this surface is chemically        treated (e.g., with a strong oxidizing agent, such as fuming        sulfuric acid and nitric acid).    -   (e) In most of the VG-CNFs, the graphene planes or graphitic        crystallites are oriented at a non-zero angle with respect to        the fiber axis, resulting in a lower strength, modulus, thermal        conductivity, and electrical conductivity along the fiber axis        direction as compared with fibrils having all graphene planes        substantially parallel to the fiber axis (e.g., CNTs).

Hence, it is desirable to have a sub-micron-scaled carbon or graphitefiber or a lower-micron graphitic fibril (diameter<6 μm, but ≧1 μm) thathas a well-controlled, consistent, and stable structure to ensureconsistent properties and performance. It is further desirable to have alow-cost process that is capable of producing graphitic fibrils in largequantities. It is also desirable to have graphitic fibrils that are pureand catalyst-free. For composite material applications, it is stillfurther desirable to have graphitic fibrils that exhibit much moresurface areas for chemical functionalization or interactions with achemical species or a matrix material. It is most desirable to have agraphitic fibril that exhibits a higher strength, modulus, thermalconductivity, and electrical conductivity as compared with conventionalCNFs.

For lithium ion battery anode applications, it is desirable to havegraphitic fibrils that have proper diameters, sufficiently small toensure easy migration of lithium ions in and out of the fibrils toenable high-rate capability, yet sufficiently small to ensure a minimalamount of solid-electrolyte interface (SEI) that would irreversiblyconsume lithium initially stored in the cathode. The main object of thepresent invention is to provide submicron graphitic fibrils that exhibitthese desirable attributes. It is another object of the presentinvention to provide lower-micron graphitic fibrils (≧1 μm, but <6 μm)that have desirable characteristics to serve as an anode active materialfor the lithium ion battery.

Concerns over the safety of earlier lithium metal secondary batteriesled to the development of lithium ion secondary batteries, in which purelithium metal sheet or film was replaced by carbonaceous materials (softcarbon, hard carbon, graphite, carbon/graphite fibers, etc.) as theanode. A graphite or carbon material can be intercalated with lithiumand the resulting graphite intercalation compound may be expressed asLi_(x)C₆, where x is typically less than 1. In order to minimize theloss in energy density, x in Li_(x)C₆ must be maximized and theirreversible capacity loss Q_(ir) in the first charge/discharge cycle ofthe battery must be minimized.

Carbon or graphite anodes can have a long cycle life due to the presenceof a protective surface-electrolyte interface layer (SEI), which resultsfrom the reaction between lithium and the electrolyte during the firstseveral cycles of charge-discharge. The lithium in this reaction comesfrom some of the lithium ions originally stored in the cathode intendedfor the charge transfer purpose. As the SEI is formed, the lithium ionsbecome part of the inert SEI layer and become irreversible, i.e, theycan no longer be the active element for charge transfer. Therefore, itis desirable to use a minimum amount of lithium for the formation of aneffective SEI layer. In addition to SEI formation, Q_(ir) of naturalgraphite has been attributed to graphite exfoliation caused byelectrolyte solvent co-intercalation and other side reactions. In theprior art, in order to prevent such an electrolyte-induced exfoliation,particles of natural graphite are typically coated with a layer ofamorphous carbon. However, such a coating effectively reduces theproportion of graphite for reversibly storing lithium. It may be notedthat the specific capacity of an anode is calculated according to thelithium storage capacity divided by the total anode weight, which is thesum of the anode active material, active material surface coating,conductive filler, and binder weights. If the amount of non-activematerials (surface coating, filler, and binder) can be reduced oreliminated, the proportion of anode active material in the anode can besignificantly increased.

The maximum amount of lithium that can be reversibly intercalated intothe interstices between graphene planes of a perfect graphite crystal isgenerally believed to occur in a graphite intercalation compoundrepresented by Li_(x)C₆ (x=1), corresponding to a theoretical specificcapacity of 372 mAh/g. In other graphitized carbon materials than puregraphite crystals, there exists a certain amount of graphitecrystallites dispersed in or bonded by an amorphous or disordered carbonmatrix phase. This amount of disordered carbon material reduces theeffective lithium storage capacity to typically <360 mAh/g, moretypically <350 mAh/g, and, in many cases, <330 mAh/g (e.g., meso-carbonmicro-beads, MCMBs, a very commonly used anode active material forelectric vehicle power applications).

Hence, it is desirable to have a new graphitic material that has amaximum amount of perfect graphene crystal structure (with a minimumamount of disordered structure), requiring no external coating andconductive additive, having an optimal diameter (small enough to enableeasy lithium entry and extraction, but large enough to have a minimalspecific surface area and, hence minimal amount of SEI), resulting inexceptional specific capacity and cycling stability. Most surprisingly,the presently invented submicron graphitic fibrils (0.1 μm<diameter<1μm) and lower-micron graphitic fibrils (1 μm≦diameter<6 μm), prepared byintercalating, exfoliating, and separating graphite or carbon fibers,exhibit all of the above desirable characteristics. This has not beentaught or implied in the prior art.

SUMMARY OF THE INVENTION

The present invention provides a submicron-scaled graphitic fibrilhaving a diameter or thickness less than 1 μm. In another preferredembodiment, the present invention also provides lower-micron graphiticfibrils (1 μm≦diameter<6 μm), particularly for use as a lithium ionbattery anode material. The fibril is typically free of continuousthermal carbon overcoat, free of continuous hollow core, and free ofcatalyst. The fibril is obtained by splitting a micron-scaled carbonfiber or graphite fiber (with a diameter typically >6 μm, but mosttypically ˜12 μm) along the fiber axis direction to form an aggregate ofstill interconnected or partially bonded fibrils, followed by separatingor isolating these fibrils from one another. This final separation orisolation step is critical to the preparation of graphitic fibrils.Un-separated or un-isolated structures are considered an “exfoliatedfiber,” which has vastly different properties from the presentlyinvented isolated graphitic fibrils. The splitting procedure can beconveniently accomplished thermo-chemically in an environmentallyfriendly manner. The separation or isolation procedure can be conductedusing a carefully controlled mechanical cutting or shearing procedure.The diameter or thickness of the resulting fibrils is typically between100 nm and 1 μm. For use as a lithium ion battery anode active material,the fibrils preferably have a diameter greater than 500 nm. If theoriginal carbon or graphite fiber is only lightly intercalated andlightly exfoliated, the resulting graphitic fibrils (after theseparation treatment) could have a diameter equal to or greater than 1μm, but less than 6 μm (mostly less than 4 μm). This is herein referredto as lower-micron graphitic fibrils.

These fully isolated graphitic fibrils exhibit exceptionally highelectrical conductivity, thermal conductivity, elastic modulus, andstrength. These are highly desirable features when they are used in thefollowing applications:

-   -   (1) Graphitic fibrils can be used as a reinforcement filler in a        structural composite or as a conductive additive in an        electrically conductive composite for static charge dissipation,        lightning strike protection, and shielding against        electromagnetic interference (EMI) or radio frequency        interference (RFI).    -   (2) The high thermal conductivity makes this class of graphitic        fibers an outstanding material for thermal management        applications.    -   (3) Multiple graphitic fibrils may be fabricated into a paper,        thin-film, mat, or web form for various engineering applications        (e.g., as a filter or membrane).    -   (4) Rubbers or tires containing these graphitic fibrils exhibit        a good stiffening effect and improved heat-dissipating        capability.    -   (5) These fibrils are also good electrode materials for energy        conversion or storage devices, such as fuel cells (using        graphitic fibrils as an ingredient in a gas diffuser plate, a        conductive additive in a bipolar plate, or a substrate to        support electro-catalyst particles), lithium-ion batteries        (e.g., using graphitic fibrils as an anode active material), and        supercapacitors (using graphitic fibrils as an electrode        material).    -   (6) These conductive fibrils are also excellent additives for        adhesives, inks, coatings, paints, lubricants, and grease        products.    -   (7) The high proportion of chemically active edge surfaces of        graphitic fibrils makes them good materials for environmental        applications (e.g., as an agent to capture heavy metal ions such        as cadmium in waste water stream and to capture oil in a spill        situation), for sensor applications (e.g., as a sensing element        for detecting biological agents), and as a filter or membrane        material.

The graphitic fibrils are preferably produced from a micron-diametercarbon fiber or graphite fiber, which is prepared from pitch,polyacrylonitrile (PAN), or rayon. Most of the conventional carbon orgraphite fibers have a diameter of approximately 12 μm, but some have adiameter as small as 6 μm. The graphitic fibril production methodpreferably comprises: (a) introducing an intercalating agent intointer-crystallite spaces or imperfections of a micron-diameter carbonfiber or graphite fiber to form an intercalated fiber; (b) activatingthe intercalating agent to split the fiber into an aggregate ofinterconnected or partially bonded submicron graphitic fibrils havinginterconnections between fibrils; and (c) mechanically severing theinterconnections to obtain the submicron graphitic fibrils or isolatingfibrils from each other.

The step of introducing an intercalating agent may comprise chemicalintercalating, electrochemical intercalating, gaseous phaseintercalating, liquid phase intercalating, supercritical fluidintercalating, or a combination thereof. The chemical intercalating maycomprise exposing a micron-diameter carbon fiber or graphite fiber to achemical selected from sulfuric acid, sulfonic acid, nitric acid, acarboxylic acid, a metal chloride solution, a metal-halogen compound,halogen liquid or vapor, potassium permanganate, alkali nitrate, alkaliperchlorate, an oxidizing agent (such as hydrogen peroxide), or acombination thereof The carboxylic acid may be selected from the groupconsisting of aromatic carboxylic acid, aliphatic or cycloaliphaticcarboxylic acid, straight chain or branched chain carboxylic acid,saturated and unsaturated monocarboxylic acids, dicarboxylic acids andpolycarboxylic acids that have 1-10 carbon atoms, alkyl esters thereof,and combinations thereof.

For an intercalating agent, the metal-halogen compound or halogen liquidor vapor may comprise a molecule selected from bromine (Br₂), iodine(I₂), iodine chloride (ICl), iodine bromide (IBr), bromine chloride(BrCl), iodine pentafluoride (IF₅), bromine trifluoride (BrF₃), chlorinetrifluoride (ClF₃), phosphorus trichloride (PCl₃), phosphorustetrachloride (P₂Cl₄), phosphorus tribromide (PBr₃), phosphorustriiodide (PI₃), or a combination thereof.

The electrochemical intercalating may comprise using nitric acid, formicacid, or a carboxylic acid as both an electrolyte and an intercalatesource. Preferably, the step of electrochemical intercalating comprisesimposing a current, at a current density in the range of 50 to 500 A/m²,to the carbon fiber or graphite fiber which is used as an electrodematerial in the electrochemical intercalating apparatus.

The step of splitting the intercalated fiber may comprise exposing theintercalated fiber to a temperature in the range of 150° C. to 1,100° C.When the step of intercalating comprises using an acid as anintercalating agent, the step of splitting the intercalated fibertypically comprises exposing the intercalated fiber to a temperature inthe range of 600° C. to 1,100° C. When the step of intercalatingcomprises using a halogen or halogen compound as an intercalating agent,the step of splitting the intercalated fiber typically comprisesexposing the intercalated fiber to a temperature in the range of 50° C.to 350° C.

It may be noted that an exfoliated graphite fiber was disclosed by D. D.L. Chung, in “Exfoliated Graphite Fibers and Associated Method,” U.S.Pat. No. 4,915,925 (Apr. 10, 1990). However, according to Chung,“Graphite fibers are exfoliated to produce a fiber of reduced density,increased diameter, and flexibility with respect to graphite fibersprior to exfoliation” (see Abstract of U.S. Pat. No. 4,915,925). Chungdid not expressly disclose or implicitly suggest the production ofgraphitic nano-fibers from the exfoliated graphite fibers via mechanicalshearing or cutting the interconnections between graphitic nano-fibrilsto isolate and separate nano-fibrils from one another. Chung did notrecognize or realize the significance of isolated/separated graphiticfibrils, which are fundamentally distinct in structure than theexfoliated fiber per se and, hence, have dramatically differentproperties. As a matter of fact, the objective of Chung's process ofexfoliation was to reduce the density and increase the diameter of thegraphite fibers, which were completely opposite to the objectives of thepresent invention. Our main objective was to extract submicron fibrilsfrom the internal structure of a carbon or graphite fiber and thesesubmicron graphitic fibrils have higher density and much lower diameters(mostly between 100 nm and 1 μm) than the original carbon or graphitefiber.

Exfoliation of carbon fibers was also studied by M. Toyoda, et al.:“Exfoliation of Carbon Fibers through Intercalation CompoundsSynthesized Electrochemically,” Carbon, 39 (2001) 1697-1707;“Intercalation of Nitric Acid into Carbon Fibers,” Carbon, 39 (2001)2231-2237; “Intercalation of Formic Acid into Carbon Fibers and theirExfoliation,” Synthetic Metals, 130 (2002) 39-43; “Exfoliation of NitricAcid Intercalated Carbon Fibers,” Carbon, 41 (2003) 731-738;“Exfoliation of Carbon Fibers,” Journal of Physics and Chemistry ofSolids, 65 (2004) 109-117; “Preparation of Intercalation Compounds ofCarbon Fibers through Electrolysis Using Phosphoric Acid Electrolyte andtheir Exfoliation,” Journal of Physics and Chemistry of Solids, 67(2006) 1178-1181; “Study of Novel Carbon Fiber Composite Used ExfoliatedCarbon Fibers,” Materials Science and Engineering, B 161 (2009) 202-204.Again, just like Chung, Toyoda et al did not expressly disclose orimplicitly suggest the production of graphitic nano-fibers fromexfoliated graphite fibers via mechanical shearing or cutting of theinterconnections between the constituent fibrils of an exfoliated carbonfiber. Toyoda et al did not recognize or realize the significance ofisolated graphitic fibrils. Again, the isolation or separation of theinterconnected constituent fibrils in an exfoliated or split carbonfiber is a critical step in the production of the presently inventedsub-micron graphitic fibrils. This critical step was not taught ineither Chung or Toyoda, et al. or their combinations.

Further, Toyoda et al exfoliated carbon fibers as a means ofaccelerating the graphitization of carbon fibers, as disclosed in“Acceleration of Graphitization in Carbon Fibers through Exfoliation,”Carbon, 42 (2004) 2567-2572. Similarly, Zhang et al split carbon fibersto facilitate the graphitization procedure, as disclosed in F. Zhang, etal. “Effect of Fiber Splitting on the Catalytic Graphitization ofElectroless Ni—B-Coated Polyacrylonitrile-Based Carbon Fibers,” Surface& Coating Technology, 203 (2008) 99-103. Neither case was directed atcreating isolated/separated graphitic fibrils.

It may be further noted that the instant applicants have previouslydisclosed a nano-scaled graphene platelet (NGP) having a thickness nogreater than 100 nm and a length-to-width ratio no less than 3(preferably greater than 10) from a carbon or graphite fiber using aseemingly similar but actually distinct process [Zhamu et al.,“Nano-scaled graphene platelets with a high length-to-width aspectratio,” US Pub. No. 2009/0155578 (Jun. 18, 2009)]. The NGP with a highlength-to-width ratio was prepared by using a method comprising (a)intercalating a carbon fiber or graphite fiber with an intercalate toform an intercalated fiber; (b) exfoliating the intercalated fiber toobtain an exfoliated fiber comprising graphene sheets or flakes; and (c)separating the graphene sheets or flakes to obtain nano-scaled grapheneplatelets (NGPs). Step (a) in our earlier disclosure was typicallycarried out to the extent that the intercalating agent significantlypenetrates the bulk of the graphite crystallites so that the subsequentexfoliation step produced ultra-thin nano graphene sheets or NGPs thatare typically thinner than 100 nm (hence, qualified as a nano-material),but more typically thinner than 1 nm (as indicated in claim 6 of US2009/0155578.

In contrast, the process of the instant application involvesintercalating a carbon fiber or graphite fiber to the extent that theintercalating agent penetrates into primarily the inter-crystallitezones or imperfections in a carbon or graphite fiber so that thesubsequent high-temperature exposure or pressure change step acts tosplit the fiber into multiple interconnected submicron graphitic fibrilshaving interconnections between the fibrils. The interconnected fibrilstypically have a diameter or thickness in the range of 100 nm(exclusive) to 1 μm (exclusive). The subsequent mechanical cutting stepserves to sever the interconnections to recover the submicron graphiticfibrils. Operationally, it might be possible or even desirable for anintercalating agent to penetrate into some interstitial spaces betweengraphene planes, but this was not a necessary condition for theproduction of the presently invented graphitic fibrils.

Perhaps, the most significant observation is the notion that thesub-micron graphitic fibers or fibrils of the instant application have adiameter greater than 100 nm and less than 1 μm and surprisingly, thisnew class of graphitic fibrils exhibit dramatically different propertiesthan NGPs. For examples, sub-micron graphitic fibrils are an excellentanode active material for lithium ion batteries, but not NGPs. Thissurprising result is experimentally demonstrated in the examples of theinstant application.

The present invention also provides a nanocomposite material thatcontains submicron graphitic fibrils. The matrix material can beselected from a polymer, rubber, plastic, resin, glass, ceramic, carbon,metal, organic, or a combination thereof. These nanocomposite materialsexhibit many unique and desirable properties.

Other embodiments of the present invention include products that containsubmicron graphitic fibrils: (a) paper, thin-film, mat, and web products(e.g., as a filter or membrane material); (b) rubber or tire products;(c) energy conversion or storage devices, such as fuel cells (as aningredient in a bipolar plate or a gas diffuser plate, or as a substrateto support electro-catalyst particles for PEM fuel cells), lithiumbatteries and supercapacitors (e.g., as an electrode active material, aconductive additive, or a current collector ingredient in asupercapacitor, lithium ion battery, lithium metal battery, and/orlithium-air battery); (d) adhesives, inks, coatings, paints, lubricants,and grease products; (e) heavy metal ion scavenger; (f) absorbent (e.g.,to recover spill oil); (g) sensors; (h) friction and brake components;and (i) high-energy radiation shielding components.

In one preferred embodiment, the present invention provides a lithiumion battery anode material comprising a submicron-scaled graphiticfibril, wherein the fibril has a diameter or thickness in the range from100 nm to 1 μm and the fibril is obtained by splitting and separating amicron-diameter carbon fiber or graphite fiber. Preferably, the fibrildiameter or thickness is in the range from 500 nm to 1 μm. Alsopreferably, the fibril has a length greater than 10 μm.

The fibril is obtained by chemically or thermo-chemically splitting acarbon or graphite fiber into an exfoliated or split fiber (an aggregateof multiple interconnected or partially bonded fibrils) and thenseparating the graphitic fibril from the aggregate. With such aproduction process, the fibril comprises at least 95% by weight carbonatoms arranged in a stack of multiple layers of hexagonal carbon orgraphene plane. Typically, the fibril has an elongate axis and comprisesa graphite single crystal having multiple layers of graphene planeparallel to the fibril elongate axis. In some cases, the fibrilcomprises multiple single crystals having multiple layers of grapheneplane. The fibrils are prepared from a micron-diameter carbon fiber orgraphite fiber having a diameter greater than or equal to 6 μm (but, inmost cases, the diameter is approximately 12 μm).

The submicron graphitic fibril is preferably obtained by a methodcomprising (a) introducing an intercalating agent into inter-crystallitespaces and/or imperfections of a micron-diameter carbon fiber orgraphite fiber to form an intercalated fiber; (b) activating theintercalating agent to split the intercalated fiber into an exfoliatedfiber (which is an aggregate of multiple interconnected submicrongraphitic fibrils having interconnections between fibrils); and (c)mechanically severing the interconnections to obtain the submicrongraphitic fibril. Typically, the intercalating agent further penetratesan inter-graphene plane space. The step of introducing an intercalatingagent may be selected from the group of chemical intercalating,electrochemical intercalating, gaseous phase intercalating, liquid phaseintercalating, supercritical fluid intercalating, and combinationsthereof.

The step of chemical intercalating comprises exposing the carbon fiberor graphite fiber to a chemical selected from the group consisting ofsulfuric acid, sulfonic acid, nitric acid, a carboxylic acid, a metalchloride solution, a metal-halogen compound, halogen liquid or vapor,potassium permanganate, alkali nitrate, alkali perchlorate, an oxidizingagent, and combinations thereof. The carboxylic acid may be selectedfrom the group consisting of aromatic carboxylic acid, aliphatic orcycloaliphatic carboxylic acid, straight chain or branched chaincarboxylic acid, saturated and unsaturated monocarboxylic acids,dicarboxylic acids and polycarboxylic acids that have 1-10 carbon atoms,alkyl esters thereof, and combinations thereof.

The metal-halogen compound or halogen liquid or vapor may comprise amolecule selected from bromine (Br₂), iodine (I₂), iodine chloride(ICl), iodine bromide (IBr), bromine chloride (BrCl), iodinepentafluoride (IF₅), bromine trifluoride (BrF₃), chlorine trifluoride(ClF₃), phosphorus trichloride (PCl₃), phosphorus tetrachloride (P₂Cl₄),phosphorus tribromide (PBr₃), phosphorus triiodide (PI₃), or acombination thereof.

The step of electrochemical intercalating comprises using a carboxylicacid as both an electrolyte and an intercalating agent source. The stepof electrochemical intercalating may comprise imposing a current, at acurrent density in the range of 50 to 600 A/m², to the carbon fiber orgraphite fiber which is used as an electrode material of anelectrochemical apparatus.

The step of activating comprises exposing the intercalated fiber to atemperature in the range of 150° C. to 1,200° C. If the step ofintroducing an intercalating agent comprises using an acid as anintercalating agent, then the step of activating may comprise exposingthe intercalated fiber to a temperature in the range of 600° C. to1,100° C.

In one preferred embodiment, the present invention provides an anodematerial containing a graphitic fibril which is prepared by using amethod comprising (a) intercalating a carbon fiber or graphite fiberwith an intercalating agent to form an intercalated fiber; and (b)thermally and/or chemically splitting or exfoliating the intercalatedfiber to obtain said graphitic fibril.

In another preferred embodiment, the anode material contains a fibrilthat is prepared by using a method comprising (a) intercalating a carbonfiber or graphite fiber with a supercritical fluid to form a tentativelyintercalated fiber at a first temperature and a first pressure; and (b)exposing the tentatively intercalated fiber to a second temperature or asecond pressure to obtain said graphitic fibril.

The present invention also provides a lithium ion battery containing theanode material described above. In a further preferred embodiment, thegraphitic fibril is coated with a thin layer of amorphous carbon afterthe splitting or separating step.

In another preferred embodiment, the present invention provides alithium ion battery anode material comprising a low-micron-scaledgraphitic fibril, wherein the fibril has a diameter or thickness greaterthan or equal to 1 μm but less than 6 μm and the fibril is obtained bysplitting and separating a carbon fiber or graphite fiber of at least 6μm in diameter. Low-micron graphitic fibrils are typically obtained fromlightly intercalated carbon or graphite fibers. In other words, thedegree of chemical intercalation and/or oxidation is typically lowerthan that is required for the production of sub-micron graphiticfibrils. If the degree of intercalation and/or oxidation is comparableto what is required of submicron fibrils, the subsequent thermalexfoliation temperature can be lower (e.g. lower than 600° C., asopposed to typically lower than 1,200° C. but higher than 600° C.).

The fibril diameter or thickness is preferably in the range from 2 μm to5 μm. The fibril preferably has a length greater than 10 μm.

The fibril may be obtained by chemically or thermo-chemically splittinga carbon or graphite fiber into an aggregate of multiple interconnectedor partially bonded fibrils and separating said graphitic fibril fromsaid aggregate. Specifically, the graphitic fibril may be obtained by amethod comprising (a) introducing an intercalating agent intointer-crystallite spaces and/or imperfections of a micron-diametercarbon fiber or graphite fiber to form an intercalated fiber; (b)activating the intercalating agent to split the intercalated fiber intoan aggregate of multiple interconnected submicron graphitic fibrilshaving interconnections between fibrils; and (c) mechanically severingthe interconnections to obtain the lower-micron graphitic fibril.

The intercalating agent can further penetrate an inter-graphene planespace. The step of introducing an intercalating agent may be selectedfrom the group consisting of chemical intercalating, electrochemicalintercalating, gaseous phase intercalating, liquid phase intercalating,supercritical fluid intercalating, and combinations thereof. The step ofchemical intercalating may comprise exposing the carbon fiber orgraphite fiber to a chemical selected from the group consisting ofsulfuric acid, sulfonic acid, nitric acid, a carboxylic acid, a metalchloride solution, a metal-halogen compound, halogen liquid or vapor,potassium permanganate, alkali nitrate, alkali perchlorate, an oxidizingagent, and combinations thereof The step of electrochemicalintercalating may comprise using a carboxylic acid as both anelectrolyte and an intercalating agent source.

The step of activating may comprise exposing the intercalated fiber to atemperature in the range of 150° C. to 1,200° C., but preferably lessthan 600° C. If the step of introducing an intercalating agent comprisesusing an acid as an intercalating agent, then the step of activatingcomprises exposing the intercalated fiber to a temperature preferably inthe range of 200° C. to 600° C. An exfoliation temperature higher than600° C. tends to produce sub-micron, instead of lower-micron graphiticfibrils.

In one preferred embodiment, the present invention provides an anodematerial containing a fibril that is prepared by using a methodcomprising (a) intercalating a carbon fiber or graphite fiber with anintercalating agent to form an intercalated fiber; and (b) thermallyand/or chemically splitting or exfoliating the intercalated fiber toobtain the graphitic fibril.

In particular, the fibril may be prepared by using a method comprising(a) intercalating a carbon fiber or graphite fiber with a supercriticalfluid to form a tentatively intercalated fiber at a first temperatureand a first pressure; and (b) exposing the tentatively intercalatedfiber to a second temperature or a second pressure to obtain thegraphitic fibril.

The present invention further provides a lithium ion battery containingthe anode material as described above. The fibril may be coated with athin layer of amorphous carbon after said splitting or separating step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Schematic of a carbon or graphite fiber consisting of multiplegraphitic fibrils, which are bounded by inter-fibril spaces orimperfections (equivalent to grain boundaries). Each graphitic fibrilcomprises a graphite single crystal or crystallite having multiplegraphene planes stacked or bonded together via van der Waals forcesalong the crystallographic c-axis direction.

FIG. 2 SEM images of (a) examples of milled carbon fibers (as a startingmicron-diameter fiber); (b) chemically intercalated carbon fibers (someinter-fibril zones or imperfections are discernible); (c) a split carbonfiber (from upper left corner to the lower right corner of the SEMimage) having interconnected or partially bonded graphitic fibrils; (d)un-separated, multiple graphitic fibrils split from another carbonfilament; and (e) three sets of split but un-separated graphitic fibrilsfrom three carbon fiber segments.

FIG. 3 Schematic of an apparatus for electrochemical intercalation ofcarbon or graphite fibers.

FIG. 4 (A) flexural strength and (B) flexural modulus of compositematerials containing graphitic fibrils from graphite fibers,multi-walled carbon nanotubes (MW-CNTs), and vapor-grown carbonnanofibers (VG-CNFs).

FIG. 5 Thermal conductivity of nanocomposites containing graphiticfibrils and VG-CNFs.

FIG. 6 Electrical conductivity of printed traces containing graphiticfibrils, MW-CNTs, and VG-CNFs.

FIG. 7 SEM images of (a) examples of split graphite fibers (withoutmechanical shearing); (b) an isolated graphitic fibril; (c) exfoliatedgraphite (without mechanical shearing to break up the graphene sheets);and (d) graphene sheets (NGPs) Obtained by isolating graphene sheets viamechanical shearing of exfoliated graphite.

FIG. 8 (a) flexural strength and (B) flexural modulus of compositematerials containing split fibers (composed of interconnected fibrils)and the composites containing isolated graphitic fibrils.

FIG. 9 Reversible specific capacity values of sub-micron graphiticfibrils (diameter>100 nm, but <1 μm) of the present invention and thoseof nano graphene platelets (thickness<100 nm), plotted as a function ofthe number of charge/discharge cycles. These data further prove that thepresently invented graphitic fibrils and NGPs are two distinct classesof materials.

FIG. 10 Reversible specific capacity values of sub-micron graphiticfibrils (diameter>100 nm, but <1 μm) of the present invention and thoseof natural graphite and carbon-coated natural graphite, plotted as afunction of the number of charge/discharge cycles.

FIG. 11 Reversible specific capacity values of sub-micron graphiticfibrils (diameter>100 nm, but <1 μm) and lower-micron graphitic fibrils(diameter≧1 μm, but <6 μm) of the present invention and those ofcarbon-coated natural graphite and meso-carbon micro-beads (MCMBs),plotted as a function of the number of charge/discharge cycles for threecharge rates (0.1 C, 1 C, and 5 C, respectively).

FIG. 12 Reversible specific capacity values of low-micron graphiticfibrils (diameter≧1 μm, but <6 μm) of the present invention and those ofthe original graphite fibers (un-exfoliated) and exfoliated graphitefibers (non-separated). The data further proves that non-separated,exfoliated carbon/graphite fibers are patently distinct and differentfrom the separated graphitic fibrils of the instant application.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Carbon materials can assume an essentially amorphous structure (glassycarbon), a highly organized crystal (graphite crystallite), or a wholerange of intermediate structures that are characterized in that variousproportions and sizes of graphite crystallites and defects are dispersedin a non-crystalline matrix. Typically, a graphite crystallite iscomposed of a number of basal planes (also referred to as grapheneplanes or graphene sheets) that are bonded together through van derWaals forces in the crystallographic c-axis direction, the directionperpendicular to the basal plane. These graphite crystallites typicallyhave one dimension on the submicron or nanometer scale. The graphitecrystallites are dispersed in or connected by crystal defects(imperfections) or an amorphous phase in a natural graphite particle.The graphite crystallites in a carbon or graphite fiber, schematicallyshown in FIG. 1, are typically elongated fibrils having a length greaterthan 10 μm and a lateral dimension (thickness or diameter) less than 1μm (hence, the name “submicron” fibrils). This lateral dimension istypically between 100 nm and 1 μm, but more typically between 100 nm and500 nm (some fibrils with a diameter<100 nm were also observed).

The present invention provides a method of extracting or isolating thesesubmicron graphitic fibrils from a micron-diameter carbon or graphitefiber (typically 6-12 μm in diameter). These graphitic fibrils, whensuccessfully isolated, provide essentially perfect graphite singlecrystals with the graphene planes (hexagon carbon structures with sp²electron hybridization) already pre-oriented parallel to the fibril axisdirection. We have most surprisingly observed that these submicrongraphitic fibrils have properties comparable to those of multi-walledCNTs and superior to those of VG-CNFs (further discussed in a latersection).

A carbon or graphite fiber is obtained from a precursor, such aspolyacrylonitrile (PAN), petroleum or coal tar pitch, or rayon. Theproduction of PAN-based carbon fibers involves oxidation of PAN fibersand carbonization of the resulting oxidized fibers at a temperaturetypically from 350° C. to 2,500° C. Pitch-based carbon fibers areobtained by heat-treating a pitch precursor to form a meso-phase orliquid crystalline phase. The procedure is followed by fiber spinningand carbonization. Both PAN- and pitch-based carbon fibers can befurther heat-treated or graphitized at a temperature of 2,500° C. to3,000° C. to obtain graphite fibers. Graphite fibers are characterizedby having more perfect and larger graphite crystallites, which arebetter oriented along the fiber axis direction, compared to carbonfibers. In the field of composite materials, many workers do notdistinguish the term carbon fiber from graphite fiber. It may be notedthat the graphite crystallites or graphitic fibrils in a graphite fiberare no different in nature from those in a carbon fiber, with theexception that the fibrils in a highly graphitized fiber are normallylarger in dimensions.

One preferred embodiment of the present invention is a submicron-scaledgraphitic fibril having a diameter or thickness less than 1 μm, whereinthe fibril is free of continuous thermal carbon overcoat, free ofcontinuous hollow core, and free of catalyst. The fibril is obtained bysplitting a micron-scaled carbon fiber or graphite fiber along the fiberaxis direction. No catalyst is used, and no expensive and slow process,such as chemical vapor deposition (CVD), catalytic chemical vapordeposition (CCVD), laser ablation, or plasma arc discharge, is needed.The diameter or thickness is preferably less than 500 nm and greaterthan 100 nm. With a thickness greater than 100 nm, the fibrils wouldhave >300 layers of graphene plane stacked together.

Most of the graphitic fibrils have a length greater than 5 μm, morecommonly greater than 10 μm, even more commonly greater than 50 μm, butcan be several hundreds of μm. Elemental and X-ray diffraction analysesof these graphitic fibrils indicate that they contain at least 95% byweight carbon atoms arranged in a hexagon or graphene structure. Mostcommonly, these fibrils comprise at least 98% by weight carbon atomsarranged in a hexagon or graphene structure.

It is of significance to point out that the original micron-diametercarbon fiber or graphite fiber contains therein very manyinter-crystallite zones or imperfections, which are the weak links inthe structure. In contrast, by separating or isolating the graphiticfibrils from one another to become individual entities, these fibrilsare essentially defect-free, single crystal type structures. Thesesingle crystals should then have ultra-high strength and elasticmodulus. Further, these fibrils are composed of essentially perfectstacks of graphene planes aligned in the fibril axis direction andgraphene is known to have exceptional electrical and thermalconductivity. Hence, the isolated graphitic fibrils should exhibitexcellent thermal and electrical conduction behaviors. We have conducteddiligent research work and experimentally proven that this indeed is thecase.

Another preferred embodiment of the present invention is a method ofproducing submicron graphitic fibrils. The method comprises: (a)introducing an intercalating agent into inter-crystallite spaces orimperfections of a micron-diameter carbon fiber or graphite fiber toform an intercalated fiber; (b) activating the intercalating agent tosplit the intercalated fiber into multiple interconnected submicrongraphitic fibrils having interconnections between fibrils; and (c)mechanically severing the interconnections to obtain the submicrongraphitic fibrils.

It may be noted that if the degree of intercalation and/or the thermalexfoliation (activation) temperature is intentionally made lower thanwhat is required of sub-micron graphitic fibrils, we tend to obtainlower-micron graphitic fibrils, instead of submicron fibrils.

FIG. 2 provides SEM images of some examples to illustrate this processfor producing graphitic fibrils. FIG. 2( a) shows some milled carbonfibers with a diameter of approximately 12 μm. FIG. 2( b) shows thechemically intercalated version of these carbon fibers (someinter-fibril zones or imperfections are discernible). The diameterremains unchanged since the intercalating agent penetrates into theimperfection or inter-graphitic fibril zones only. This was furtherconfirmed by X-ray diffraction data, which indicate that the diffractionpeak at 2θ=26°, corresponding to an inter-planar spacing of d=0.335 nm,remains unchanged after intercalation. If the intercalating agent hadsignificantly penetrated the inter-graphene spacing, the 2θ would havedecreased to approximately 13°, corresponding to an inter-planar spacingof d=0.55-0.65 nm commonly found in a graphite intercalation compound.

FIG. 2( c) shows a split carbon fiber (which is positioned from theupper left corner to the lower right corner of the SEM image) havinginterconnected graphitic fibrils. FIG. 2( d) shows multiple graphiticfibrils split from another carbon filament and FIG. 2( e) shows threesets of split but un-separated graphitic fibrils from three carbon fibersegments.

The step of introducing an intercalating agent may comprise chemicalintercalating, electrochemical intercalating, gaseous phaseintercalating, liquid phase intercalating, supercritical fluidintercalating, or a combination thereof. The chemical intercalating maycomprise exposing the carbon fiber or graphite fiber to an intercalate(intercalating agent or intercalant) selected from sulfuric acid,sulfonic acid, nitric acid, a carboxylic acid, a metal chloridesolution, a metal-halogen compound, halogen liquid or vapor, potassiumpermanganate, alkali nitrate, alkali perchlorate, an oxidizing agent, ora combination thereof. The carboxylic acid may be selected from thegroup consisting of aromatic carboxylic acid, aliphatic orcycloaliphatic carboxylic acid, straight chain or branched chaincarboxylic acid, saturated and unsaturated monocarboxylic acids,dicarboxylic acids and polycarboxylic acids that have 1-10 carbon atoms,alkyl esters thereof, and combinations thereof. Alternatively, theintercalant may comprise an alkali metal (e.g., Li, Na, K, Rb, Cs, or acombination thereof, such as a eutectic).

For an intercalating agent, the metal-halogen compound or halogen liquidor vapor may comprise a molecule selected from bromine (Br₂), iodine(I₂), iodine chloride (ICl), iodine bromide (IBr), bromine chloride(BrCl), iodine pentafluoride (IF₅), bromine trifluoride (BrF₃), chlorinetrifluoride (ClF₃), phosphorus trichloride (PCl₃), phosphorustetrachloride (P₂Cl₄), phosphorus tribromide (PBr₃), phosphorustriiodide (PI₃), or a combination thereof.

In the case of a chemical intercalation (using an acid as anintercalant, for instance), the method comprises:

(a) forming an acid-intercalated fiber by a chemical intercalationreaction which, as examples, uses a combination of a sulfuric acid andnitric acid, or a combination of carboxylic acid and hydrogen peroxide,as an intercalate source. In these two examples, sulfuric acid orcarboxylic acid serves as an intercalating agent while nitric acid orhydrogen peroxide serves as an oxidizing agent in anintercalant-oxidizer mixture. The carbon or graphite fiber is simplyimmersed in such a mixture at a desired temperature (typically 20-80°C.) for a length of time sufficient for effecting the intercalationreaction. It may be noted that if the intercalation time is sufficientlyshort (e.g. <30 minutes in a mixture of sulfuric acid and nitric acid),intercalation appears to be limited to the inter-crystallite spaces orimperfections only with little acid penetrating into inter-grapheneplane spaces. Longer intercalation times usually led to penetration ofintercalants into both inter-crystallite and inter-graphene spaces. As aresult, one tended to obtain ultra-thin nano graphene platelets with athickness <100 nm, as observed and reported by the applicants earlier[Zhamu et al., US Pub. No. 2009/0155578, Jun. 18, 2009]);

(b) rapidly heating the intercalated carbon or graphite fiber to a hightemperature and allowing the fiber to stay at this temperature for adesired length of time sufficient for expanding the intercalating agent(sulfuric acid being decomposed to produce highly volatile gases thatact to push apart graphitic fibrils), thereby producing a split fiber(e.g., 650° C.-1,100° C. in the cases where a mixture of an acid and anoxidizing agent is used as an intercalate). The carboxylic acidintercalation is preferred as the subsequent splitting step in this casedoes not involve the evolution of undesirable species, such as NO_(x)and SO_(x), which are common by-products of exfoliating conventionalsulfuric or nitric acid-intercalated graphite compounds; and

(c) subjecting the resulting split fiber to a mild mechanical shearingtreatment (e.g., using a rotating-blade mill, air mill, pressurized gasmill, ball mill, or untrasonicator) to sever the interconnectionsbetween fibrils to produce the desired graphitic fibrils.

The carboxylic acid, containing only C, H, and O atoms, may be selectedfrom the group consisting of aromatic carboxylic acid, aliphatic orcycloaliphatic carboxylic acid, straight chain or branched chaincarboxylic acid, saturated and unsaturated monocarboxylic acids,dicarboxylic acids and polycarboxylic acids that have 1-10 carbon atoms,alkyl esters thereof, and combinations thereof. Preferably, thecarboxylic acid is selected from the group consisting of saturatedaliphatic carboxylic acids of the formula H(CH₂)_(n)COOH, wherein n is anumber of from 0 to 5, including formic, acetic, propionic, butyric,pentanoic, and hexanoic acids, anydrides thereof, reactive carboxylicacid derivatives thereof, and combinations thereof In place of thecarboxylic acids, the anhydrides or reactive carboxylic acid derivativessuch as alkyl esters can also be employed. Representative of alkylesters are methyl formate and ethyl formate. The most preferredcarboxylic acids are formic acid and acetic acid.

Representative of dicarboxylic acids are aliphatic dicarboxylic acidshaving 2-12 carbon atoms, in particular oxalic acid, fumaric acid,malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid,1,5-pentanedicarboxylic acid, 1,6-hexanedicarboxylic acid,1,10-decanedicarboxylic acid, cyclohexane-1,4-dicarboxylic acid andaromatic dicarboxylic acids such as phthalic acid or terephthalic acid.Representative of alkyl esters are dimethyl oxylate and diethyl oxylate.Representative of cycloaliphatic acids is cyclohexane carboxylic acidand of aromatic carboxylic acids are benzoic acid, naphthoic acid,anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m- andp-tolyl acids, methoxy and ethoxybenzoic acids, acetoacetamidobenzoicacids and, acetamidobenzoic acids, phenylacetic acid and naphthoicacids. Representative of hydroxy aromatic acids are hydroxybenzoic acid,3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid,4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid,5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid and7-hydroxy-2-naphthoic acid. Among the polycarboxylic acids, citric acidis preferred due to its availability and low cost.

Carboxylic acids can be intercalated into carbon fibers both chemicallyand electrochemically. The carboxylic acid-intercalated carbon orgraphite fiber can be easily split by rapidly heating the intercalatedfiber at a desired temperature. An advantage of such a carboxylicacid-intercalated fiber in comparison with sulfuric acid-intercalatedmaterial is that only H, C and O are released into the atmosphere duringthe exfoliation process. Depending on the applied current density (inthe case of electrochemical intercalation) and the reaction time, anexpansion volume of from 30-100 ml/g, at 400-800° C., and volatilecontent of 5-15 wt %, could be obtained. Furthermore, the split fiberand subsequent graphitic fibrils do not contain additional corrosivespecies, such as chlorine, fluorine, nitrogen, and phosphor.

The mechanical shearing treatment, used to separate graphitic fibrilspreferably comprises using air milling (including air jet milling), ballmilling, mechanical shearing (including rotating blade fluid grinding),any fluid energy based high-shearing process, ultrasonication, or acombination thereof.

In the case of electrochemical intercalation, the desired graphiticfibrils may be obtained by a method comprising: (a) forming anacid-intercalated carbon or graphite fiber by an electrochemicalreaction which uses an acid (e.g., formic acid, nitric acid, or acarboxylic acid) as both an electrolyte and an intercalate source, thecarbon or graphite fiber as an anode material, and a metal or graphiteas a cathode material, and wherein a current is imposed upon the cathodeand the anode at a current density for a duration of time sufficient foreffecting the electrochemical reaction; (b) exposing the intercalatedcarbon or graphite fiber to a thermal shock to produce a split fiber;and (c) subjecting the split fiber to a mechanical shearing treatment toproduce the graphitic fibrils. The fiber splitting step preferablycomprises heating the intercalated carbon or graphite fiber to atemperature in the range of 300-1,100° C. for a duration of 10 secondsto 1 minute, most preferably at a temperature in the range of 400-600°C. for a duration of 30-60 seconds.

Schematically shown in FIG. 3 is an apparatus that can be used forelectrochemical intercalation of carbon or graphite fibers according toa preferred embodiment of the present invention. The apparatus comprisesa container 32 to accommodate electrodes and electrolyte. The anode iscomprised of multiple carbon or graphite fiber segments 40 that aredispersed in an electrolyte (e.g., a carboxylic acid, which is also anintercalating agent) and are supported by a porous anode supportingelement 34, preferably a porous metal plate, such as platinum or lead.The carbon or graphite fiber segments 40 preferably form a continuouselectron path with respect to the anode support plate 34, but areaccessible to the intercalating agent. An electrically insulating,porous separator plate 38 (e.g., Teflon fabric or glass fiber mat) isplaced between the anode and the cathode 36 (e.g., a porous graphite ormetal plate) to prevent internal short-circuiting. A DC current source46 is used to provide a current to the anode support element 34 and thecathode 36. The imposing current used in the electrochemical reactionpreferably provides a current density in the range of 50 to 600 A/m²,most preferably in the range of 100 to 400 A/m². Fresh electrolyte(intercalate) may be supplied from an electrolyte source (not shown)through a pipe 48 and a control valve 50. Excess electrolyte may bedrained through a valve 52. In a possible arrangement, the carbon orgraphite fiber segments 40 may be long or continuous-length fiber yarnsthat are used as an anode directly connected to a DC current source.

Carbon or graphite fibers also can be intercalated with an alkali metal.For instance, the fiber sample may be heated to 200° C. in an evacuatedtube in the presence of potassium to form an intercalated fiber. Theintercalated fiber is then brought in contact with a liquid, such aswater, methanol, ethanol, a hydroxylic solvent, or a solvent containingwater. Rapid splitting of the fiber can occur in ethanol, creating asplit fiber containing highly separated graphitic fibrils, which may notrequire a subsequent mechanical shearing treatment.

More specifically, a carbon or graphite fiber can be intercalated withalkali (Li, Na, K, Rb, Cs), alkaline earth (Ca, Ba, Sr), or lanthanidemetals (Eu, Yb, Sm, Tm) by five different methods: (1) The larger alkalimetals (K, Rb, Cs) intercalate the fiber readily by heating at 200° C.Lithium can be intercalated into a carbon fiber, but only at highertemperatures and/or pressures. Sodium intercalation is difficult, butcan be accomplished using high reaction temperatures (500-600° C.) forone week. Intercalation using the alkaline earth (Ca, Ba, Sr) orlanthanide metals (Eu, Yb, Sm, Tm) also requires high temperatures andlonger reaction times (similar to lithium intercalation); (2) The carbonor graphite fiber can be intercalated electrochemically using anon-aqueous solvent; (3) An alkali plus naphthalene or benzophenone canbe used with a suitable non-aqueous solvent (usually an ether such astetrahydrofuran); (4) Any of the above mentioned metals (except Li andNa) can be intercalated by dissolving in a liquid ammonia solution tocreate solvated electrons; and (5). Lithium can be intercalated into acarbon or graphite fiber by using n-butyl lithium in a hydrocarbonsolvent (e.g., hexane). The alkali metal-intercalated fibers can beimmersed in water or water-alcohol mixture to produce exfoliated andseparated graphitic fibrils. An additional mechanical shearing may notbe required.

The produced submicron or lower-micron graphitic fibrils may be used asan anode active material in a lithium ion battery. This is illustratedin several samples of Example 6 below.

The following examples serve to provide the best modes of practice forthe present invention and should not be construed as limiting the scopeof the invention:

EXAMPLE 1 Graphitic Fibrils from PAN-Based Graphite Fibers

Continuous graphite fiber yarns (Magnamite from Hercules) were cut intosegments of 5 mm long and then ball-milled for 24 hours. Approximately20 grams of these milled fibers were immersed in a mixture of 2 L offormic acid and 0.1 L of hydrogen peroxide at 45° C. for 4 hours.Following the chemical oxidation intercalation treatment, the resultingintercalated fibers were washed with water and dried. The resultingproduct is a formic acid-intercalated graphite fiber material.

Subsequently, the intercalated fiber sample was transferred to a furnacepre-set at a temperature of 600° C. for 30 seconds. The intercalatedgraphite fiber was found to undergo rapid splitting into graphiticfibrils. Further separation of graphitic fibrils from split fibers wasachieved using a Cowles shearing device. The fibril diameters were foundto be in the range of 80 nm to 250 nm.

When an exfoliation temperature of 200° C.-600° C. was used, theresulting fibrils had a diameter in the range of 0.5 μm-1 μm. When theintercalation time was 2 hours instead of 4 and the exfoliationtemperature of 200° C.-600° C. was used, the resulting fibrils had adiameter in the range of 1 μm-5.9 μm. This observation provides us witha guideline for producing submicron or lower-micron graphitic fibrils ofdesired diameters.

EXAMPLE 2 Graphitic Fibrils from Sulfuric/Nitric Acid-IntercalatedPitch-Based Carbon Fibers and Nanocomposites Containing Such Fibrils

Fifty grams each of a series of carbon and graphite fibers from Amoco(P-25, P-30X, P-55S, P-75S, P-100S, and P-120S) were intercalated with amixture of sulfuric acid, nitric acid, and potassium permanganate at aweight ratio of 4:1:0.05 (graphite-to-intercalate ratio of 1:3) for onehour. Upon completion of the intercalation reaction, the mixture waspoured into deionized water and filtered. The sample was then washedwith 5% HCl solution to remove most of the sulfate ions and residualsalt and then repeatedly rinsed with deionized water until the pH of thefiltrate was approximately 5. The dried sample was then exposed to aheat shock treatment at 600° C. for 45 seconds. These samples wereseparately submitted to a mechanical shearing treatment in a Cowles (arotating-blade dissolver/disperser) for 10 minutes. The resultinggraphitic fibrils were examined using SEM and TEM and their length anddiameter were measured. The average diameters of the resulting graphiticfibrils from P-25, P-30X, P-55S, P-75S, P-100S, and P-120S were found tobe approximately 120 nm, 155 nm, 343 nm, 456 nm, and 576 nm,respectively.

Graphitic fibrils from P-25, P-55S, and P-100S were separately mixedwith an epoxy resin to obtain nanocomposite samples of various fibrilweight fractions. For comparison purposes, multi-walled carbon nanotubes(MW-CNTs from Cheap Tubes, LLC) and vapor-grown carbon nano-fibers(VG-CNFs from Pyrograf) were also mixed with the same epoxy resin undercomparable processing conditions to form composite samples. The flexuralproperties (flexural strength and flexural modulus) and thermalconductivity values of the nanocomposite samples were measured andplotted as a function of the fibril volume fraction.

FIG. 4( a) demonstrates that the flexural strength of MW-CNT/epoxycomposites is slightly higher than that of our graphitic fibril/epoxycomposite at an equal nano-filler volume fraction when the volumefractions are relatively low (<3%). However, the strength of CNTcomposites drops significantly beyond 3% by volume of CNTs, likely dueto the difficulty in disentangling CNTs and dispersing CNTs in a resin.This was not the case for our graphitic fibril composites. The ease ofdispersing and processing is one of the surprising and highly desirablefeatures of the presently invented graphitic fibrils. Also surprisingly,the strength of graphitic fibril composites is consistently higher thanthat of VG-CNF composites. This is likely due to the better dispersionof graphitic fibrils in epoxy resin, stronger interfacial bond betweengraphitic fibrils and epoxy resin (since there is no inert thermalcarbon overcoat on our graphitic fibrils and graphene edges are directlyaccessible by the resin), and the higher strength of the graphiticfibrils (since all the graphene planes are well-aligned parallel to thefibril axis).

Possibly for these same reasons, the flexural modulus of graphiticfibril-epoxy composites is consistently higher than that of VG-CNFcomposites, as shown in FIG. 4( b). The flexural modulus of thegraphitic fibril composite is comparable to that of the CNT composite.

Even more surprisingly, as shown in FIG. 5, the thermal conductivity ofgraphitic fibril composites is significantly higher than that ofcorresponding VG-CNF composites by a big margin. This could be due tothe intrinsically higher thermal conductivity of graphitic fibrils andbetter dispersion of graphitic fibrils in a resin as compared withVG-CNFs, which tend to form an agglomerate called a “bird's nest.”

Although a resin is herein used as an example, the matrix material formaking a nanocomposite is not limited to a polymer (thermoplastic,thermoset, rubber, etc). The matrix can be a glass, ceramic, carbon,metal, or other organic material, provided the matrix material can bemixed with graphitic fibrils via melt mixing, solution mixing, vaporinfiltration, solid state sintering, etc. The weight fraction ofgraphitic fibrils in such a nanocomposite can be between 0.01% and 90%.The high fibril weight percentage nanocomposite may be produced byforming graphitic fibers into a porous paper or web form, followed byimpregnating a resin into the pores or simply immersing the paper or webin a resin solution. The resin content can be controlled to be evenlower than 10% by weight.

The production processes for carbon fiber reinforced carbon matrix,metal matrix, glass matrix, and ceramic matrix composite materials arewell-known in the art. The presently invented graphitic fibrils aresimilar in processing characteristics than short or choppedcarbon/graphite fibers. Hence, the processing methods of graphiticfibril composites are expected to be similar to those for short fibercomposites.

As one example, graphitic fibrils can be dispersed in a petroleum orcoal tar pitch, or a high carbon yield resin (such as phenolic resin,PAN, and polyfufuryl alcohol) to form a composite component. Thiscomponent can then be subjected to carbonization treatment (600-2,500°C.) to convert the resin into a carbon matrix to make a graphiticfibril-carbon matrix composite (C/C composite). A C/C composite is aparticularly useful friction product (friction component in anautomobile transmission or clutch system) or aircraft brake material.The C/C composite may be further heat-treated at a temperature higherthan 2,500 C to graphitize the carbon matrix to make a graphiticfibril/graphite composite material (Gr/Gr composite). Such a Gr/Grcomposite is of particular utility value in ultra-high temperatureapplications, e.g., as a rocket motor casing, rocket nose cone,refractory lining, and steel-making furnace electrode material. Due tothe high radiation cross-section, these high strength Gr/Gr compositescan be used for radiation-shielding applications against high-energybeams, such as neutron and Gamma radiation.

EXAMPLE 3 Graphitic Fibrils from Electrochemical Intercalation andSplitting of Carbon Fibers

In a typical experiment, one gram of P-25 fibers, ground toapproximately 220 μm in length, was used as the anode material and 1 Lof nitric acid was used as the electrolyte and intercalate source in anelectrochemical intercalation system. The anode supporting element is aplatinum plate and the cathode is a graphite plate of approximately 4 cmin diameter and 0.2 cm in thickness. The separator, a glass fiberfabric, was used to separate the cathode plate from the graphite/carbonfibers and to compress the fibers down against the anode supportingelement to ensure that the graphite/carbon fiber segments are inelectrical contact with the anode supporting element to serve as theanode. The electrodes, electrolyte, and separator are contained in aBuchner-type funnel to form an electrochemical cell. The anodesupporting element, the cathode, and the separator are porous to permitintercalate (electrolyte) to saturate the fibers and to pass through thecell from top to bottom.

The fiber segments were subjected to an electrolytic oxidation treatmentat a current of 0.5 amps (current density of about 0.04 amps/cm²) and ata cell voltage of about 4-6 volts for 30 minutes. These values may bevaried with changes in cell configuration and makeup. Followingelectrolytic treatment, the resulting intercalated fiber was washed withwater and dried.

Subsequently, approximately ⅔ of the intercalated fiber sample wastransferred to a furnace pre-set at a temperature of 600° C. for 30seconds. The intercalated fiber was found to induce rapid splitting. Anultrasonicator (operated with a power of 80 W) was used to separate thefibrils. The diameters of individual fibrils were found to range from130 to 350 nm based on SEM observations.

The suspension of graphitic fibrils dispersed in water, afterultrasonication-assisted dispersion, was sprayed (printed) onto a papersubstrate to form a line of approximately 3 mm wide and 15 μm thick.Similarly, MW-CNTs and VG-CNFs were also formed into traces ofconductive fillers. The electrical conductivity of these conductivefiller lines or traces (no binder) was measured using a four-point probemethod. The data is summarized in FIG. 6, which indicates that graphiticfibrils and CNTs have comparable electrical conductivity, which issignificantly higher than that of VG-CNFs.

EXAMPLE 4 Supercritical CO₂ Fluid Intercalation and Splitting ofGraphite Fibers

One gram of PAN-based graphite fibers (Magnamite from Hercules) wasplaced in a tube furnace at a temperature of 800° C. in a nitrogenatmosphere for 3 hours to remove the surface finish of graphite fibers.

The treated graphite fiber sample was placed in a 100 milliliterhigh-pressure, single-compartment vessel with a heating provision. Thevessel was capable of being isolated from the atmosphere by securityclamps and ring. The vessel was in fluid communication with thehigh-pressure carbon dioxide by way of piping means and limited byvalves. A heating jacket was disposed about the vessel to achieve andmaintain the critical temperature of carbon dioxide.

When the vessel was isolated, the pressurized carbon dioxide wasintroduced therein and maintained at about 1,100 psig (pressure of 76bars). Then, the vessel was heated to about 70° C. at which thesupercritical conditions of carbon dioxide were achieved and maintainedfor about 30 minutes to effect intercalation. Then, the vessel wasimmediately depressurized at a rate of about 3 milliliters per second,thus catastrophically lowering the pressure within the vessel. This wasaccomplished by opening a connected blow-off valve of the vessel. As aresult, a split graphite fiber was formed. After a mechanical shearingtreatment in a laboratory-scale Cowles rotating blade device for 15minutes, the resulting graphitic fibrils exhibit a diameter ranging from65 nm to 156 nm.

COMPARATIVE EXAMPLE 5 Un-Separated, Split Carbon Fibers and Composites

Additional amounts of thermally split graphite fibers (P-25) wereprepared according to the procedure described in Example 2 above, butthe split fibers did not go through a mechanical shearing step toisolate the graphitic fibrils. These split graphite fibers composed ofun-separated (still interconnected) fibrils are fundamentally differentmaterials from the isolated graphitic fibrils of the present invention.

The split or conventionally exfoliated carbon or graphite fibers(without a mechanical shearing treatment) are relatively soft, of lowerstrength, and lower density (approximately 1.4-2.0 g/cm³), and they areessentially a loosely connected web structure (FIG. 7 a). In contrast,the isolated fibrils (e.g., FIG. 7 b) are of exceptionally high strengthand high stiffness, and have higher physical density (approximately 2.25g/cm³).

Analogously, the exfoliated graphite (or graphite worms obtained byintercalating natural graphite to obtain a graphite intercalationcompound and then exfoliating the resulting graphite intercalationcompound) is soft, fluffy, of extremely low strength, highly porous, andcomposed of graphite flakes or graphene planes spaced apart by pores (agraphite worm is shown in FIG. 7 c). In contrast, after mechanicalshearing, one obtains nano graphene platelets, which were recently foundto exhibit the highest intrinsic strength and highest thermalconductivity of all materials known to scientists. Nano graphene is nowcommonly regarded as the most promising nano material in the scientificcommunity. These observations further assert the significance of thefinal mechanical shearing step in isolating/separating the graphiticfibrils (e.g. FIG. 7 d).

We proceeded to mix the split but un-separated graphite fibers in thesame epoxy resin as used in Example 2 to produce “split graphitefiber-reinforced composites” or “exfoliated graphite fiber-reinforcedcomposites,” as opposed to the presently invented graphiticfibril-reinforced composites. A comparison of the flexural modulus andstrength of these two classes of composite materials is given in FIGS. 8a and 8 b. These data have clearly demonstrated that the graphiticfibrils of the present invention are dramatically more effective inreinforcing a resin matrix as compared with split or exfoliated graphitefibers (without a mechanical shearing step). These highly surprisingresults could not have been anticipated and, actually, have not beenanticipated by any prior art workers. It is fair to say that, afterdiligent research and development work, we have discovered a totally newclass of high-performance materials that have tremendous utility value.This new class of materials has never been taught in the prior art andhas not been an obvious extension of any prior art work.

EXAMPLE 6 Graphitic Fibrils as a Lithium Ion Battery Anode Material

The above examples provide graphitic fibrils for use as an anode activematerial of a lithium ion battery. In this example, coin cells were usedto evaluate and compare the performance of sub-micron graphitic fibrilsas an anode material against other types of anode materials, such asnano graphene platelets (thickness<100 nm), natural graphite (un-coatedand amorphous carbon-coated), and lower-micron graphitic fibrils(diameter≧1 μm, but <6 μm) obtained by separating lightly exfoliatedgraphite fibers.

FIG. 9 shows that the presently invented sub-micron graphitic fibrils(diameter>100 nm, but <1 μm) are a fundamentally distinct class of anodeactive material than nano graphene platelets (NGPs) with a highlength-to-width ratio even though both classes of materials can beobtained from micron-scaled carbon or graphite fibers. The graphiticfibrils, when used as a lithium ion battery anode active material,exhibit not only a significantly higher reversible capacity, but alsomuch more stable cycling response. Actually, the cycling response ofgraphitic fibrils and that of NGPs are fundamentally distinct. Forinstance, the specific capacity undergoes some minor decay initially,but remains essentially unchanged after 15 cycles for all the sub-microngraphitic fibrils investigated (diameter=0.96 μm, 0.48 μm, and 0.18 μm)for a large number of subsequent cycles. In contrast, the NGPs (averagethickness=47 nm and 11 nm) show a very rapid drop in specific capacityand never reach a plateau value. This observation further asserts thatthe graphitic fibrils are a totally distinct class of material than NGPsand they are superior to NGPs when used as a lithium ion battery anodeactive material.

FIG. 10 shows that the presently invented sub-micron graphitic fibrils(diameter>100 nm, but <1 μm) are superior to natural graphite in termsof specific capacity and cycling stability. Natural graphite iscurrently the most commonly used anode material, but requires a coatingprocess to deposit a thin layer of amorphous carbon that completelyencloses the graphite particle to prevent graphene layerexfoliation-induced capacity delay. This is illustrated in the lower twocurves of FIG. 10. In contrast, and quite surprisingly, the graphiticfibrils (without any externally applied carbon coating) exhibit a verystable cycling behavior. Even though graphene planes are also the basicconstituent structure in a graphitic fibril (just as in naturalgraphite), no electrolyte-induced graphene plane exfoliation was foundto occur in the lithium ion cells featuring graphitic fibrils as theprimary anode active material. This is quite unexpected and the reasonsremain unclear at this stage.

We also proceeded to prepare a graphitic fibril anode sample thatcontains a carbon coating on the fibril surface. This was prepared bymixing the fibrils with petroleum pitch powder and then enclosing theresulting mixture in a sealed quartz tube. The tube was heated to 400°C. and then cooled back to room temperature to allow a thin layer ofpitch to embrace the graphitic fibrils. The charge-discharge cyclingbehavior was found to be somewhat better than that of the non-coatedversion. The difference was not very significant.

FIG. 11 shows the specific capacity of various anode active materials asa function of the cell charge rates (1 C=charge completed in 1 hour; 0.1C=charge completed in 10 hours; and 5 C=charge completed in ⅕ hours or12 minutes). For electric vehicle applications, the batteries must beable to charge and discharge at high rates, requiring easy lithiummigration in and out of the anode active material and cathode activematerial. The present example demonstrates that, as compared withcarbon-coated natural graphite, graphitic fibrils (either thinner than 1μm but greater than 100 nm, or slightly higher than 1 μm, but less than6 μm) exhibit much higher specific capacity values for both low and highcharge rates. Even more surprisingly, graphitic fibrils are far superiorto meso-carbon micro-beads (MCMBs) in terms of specific capacity even atvery high charge rates (e.g., 5 C). MCMBs are known to be excellentanode active materials for electric vehicle battery applications due totheir high-rate capability. The presently invented graphitic fibrils areeven better than MCMBs in terms of high-rate capability and specificcapacity for all rates.

FIG. 12 serves to demonstrate that exfoliated (split) graphite fibers,if not separated to produce isolated graphitic fibrils, exhibit acharge-discharge cycling response similar to that of the originalgraphite fiber (12 μm, non-intercalated and non-exfoliated). After themechanical separation treatment, the resulting graphitic fibrils show asignificantly higher specific capacity and more stable cycling behavior.This observation again confirms the notion that the presently inventedgraphitic fibrils are a fundamentally different and patently distinctclass of materials than the exfoliated (but un-separated) graphitefibers.

The presently invented graphitic fibrils have the following desirablefeatures and advantages when compared with carbon nanotubes (CNTs) andconventional carbon nanofibers:

-   -   (1) The presently invented graphitic fibrils can be readily        mass-produced at low costs. Millions of kilograms of carbon or        graphite fibers are produced annually at a much lower prices        (e.g., $20-30/Kg) than CNTs (e.g., currently >$150/Kg, up to        several thousands of US dollars per kilogram for high-purity        products) and CNFs (>$200/Kg). The process involves a simple        intercalation procedure, which requires a much shorter        intercalation time as compared with intercalation of graphite        particles since penetration of chemical into inter-graphene        spaces is not required. Intercalation of graphite is commonly        practiced to produce graphite worms and flexible graphite        sheets. This process is relatively low-cost, adding less than        $2-3 per kilogram for processing costs. Since our process        requires penetration of an intercalating agent into        inter-crystallite or imperfection regions of a carbon or        graphite fiber (not necessarily the inter-graphene spaces), more        environmentally benign intercalating agents can be used; e.g.,        formic acid or citric acid as opposed to sulfuric and nitric        acids.    -   (2) The submicron graphitic fibrils have a well-controlled,        consistent, and stable structure to ensure consistent properties        and performance. The structure is already highly graphitized,        requiring no further graphitization treatment. The graphitic        structure remains stable in a non-oxidizing environment up to        3,000° C.    -   (3) The presently invented graphitic fibrils are pure and        catalyst-free.    -   (4) Graphitic fibrils exhibit much more surface areas for        chemical functionalization or interactions with a chemical        species or a matrix material. This is a critically important        feature since the ability for a nano-filler surface to be easily        modified chemically or physically in a controlled manner is        essential to the success of many engineering applications. For        instance, the large graphene edge surface area makes this        graphitic fibrils readily functionalized by functional groups        (e.g., amine) that are chemically compatible or reactive with a        resin (e.g. epoxy) for composite material applications.    -   (5) The graphitic fibrils exhibit higher strength, modulus,        thermal conductivity, and electrical conductivity as compared        with conventional CNFs. These properties of graphitic fibrils        are comparable to those of CNTs.

The applicants have observed that the presently invented graphiticfibrils, when incorporated in the following products, impart highlydesirable properties to these products, including: (a) paper, thin-film,mat, and web products (e.g., as a membrane or filter material); (b)rubber or tire products (to stiffen selected portions of a tire and toaid in heat dissipation); (c) energy conversion or storage devices, suchas fuel cells, lithium-ion batteries, and supercapacitors; (d)adhesives, inks, coatings, paints, lubricants, and grease products (as aconductive additive and/or friction- or wear-reducing agent); (e) heavymetal ion scavenger; (f) absorbent; (g) sensors; (h) friction and brakecomponents; (i) radiation shield; and (j) nanocomposite materials.

Hence, other embodiments of the present invention include:

-   -   (a) A paper, thin-film, mat, web, membrane, or filter product        containing the presently invented graphitic fibril: The paper,        mat, or web containing graphitic fibrils is a good substrate        material to support electro-active materials, such as Si, in a        lithium-ion battery. They provide a protective layer on the        composite aircraft skin for lightning strike protection since        graphitic fibrils are highly conductive. These products are also        good filter or membrane materials due to the strength and        rigidity of graphitic fibrils and their ability to kill        biological agents when these fibrils are activated;    -   (b) A rubber or tire product containing the graphitic fibril: A        tire product makes use of the high elastic modulus to stiffen        selected portions of a tire and the high thermal conductivity to        aid in the heat dissipation; the driving-induced heat would        otherwise significantly increase the tire wear rate. Graphitic        fibrils significantly increase the strength and conductivity of        a rubber;    -   (c) A fuel cell bipolar plate (graphitic fibrils serving as a        conductive additive in a resin), gas diffuser plate (conductive        mat of graphitic fibrils), or electrode containing the graphitic        fibril (fibrils serving as an electrocatalyst-supporting        substrate);    -   (d) An electrochemical device: For instance, graphitic fibrils        can be used as an electrode active material, a conductive        additive, or a current collector ingredient in a supercapacitor,        lithium ion battery, lithium metal battery, and/or lithium-air        battery). When used as an anode active material, the presently        invented graphitic fibrils exhibit a significantly higher        specific capacity as compared with nano graphene platelets,        natural graphite, MCMBs, un-intercalated and un-exfoliated        graphite fibers, and exfoliated but non-separated        carbon/graphite fibers;    -   (e) An adhesive, ink, coating, paint, lubricant, or grease        product containing the graphitic fibril: These products make use        of the electrical conductivity, thermal conductivity, graphitic        plane-sliding characteristic, friction-controlling, and        wear-reducing capabilities of graphitic fibrils;    -   (f) A heavy metal ion scavenger or absorbent containing the        graphitic fibril: These applications make use of the easily        attached functional groups to capture heavy metal ions,        bacteria, oil in a stream;    -   (g) A sensor device containing the graphitic fibril as a sensing        element: The conductivity, capacitance, and other        electro-chemical characteristics of fibrils are highly sensitive        and selective to the presence of analytes, contaminants, or        other chemical and biological agents;    -   (h) A friction component or brake component containing the        graphitic fibril: The graphitic fibrils are excellent ingredient        in a friction plate commonly used in an automobile clutch or        transmission system. Graphitic fibrils incorporated in a carbon        matrix provide a good carbon-carbon composite that exhibits        outstanding braking performance in an aircraft brake        environment;    -   (i) A radiation-shield component containing the graphitic        fibril: Graphite is an excellent high-energy radiation resistant        material. By combining graphitic fibrils with a graphitized        carbon matrix, we obtain a graphite/graphite (Gr/Gr) composite        that is of high strength and high radiation resistance.

The invention claimed is:
 1. A lithium ion battery anode material comprising a submicron-scaled graphitic fibril as an anode active material, wherein said fibril has a diameter or thickness in the range from 100 nm to 1 μm and said fibril is obtained by splitting and separating a micron-diameter carbon fiber or graphite fiber.
 2. The anode material of claim 1 wherein said fibril diameter or thickness is in the range from 500 nm to 1 μm.
 3. The anode material of claim 1 wherein said fibril has a length greater than 10 μm.
 4. The anode material of claim 1 wherein said fibril is obtained by chemically or thermo-chemically splitting a carbon or graphite fiber into an aggregate of multiple interconnected or partially bonded fibrils and separating said graphitic fibril from said aggregate.
 5. The anode material of claim 1 wherein said fibril comprises at least 95% by weight carbon atoms arranged in a stack of multiple layers of hexagonal carbon or graphene plane.
 6. The anode material of claim 1 wherein said fibril has an elongate axis and comprises a graphite single crystal having multiple layers of graphene plane parallel to the fibril elongate axis.
 7. The anode fibril of claim 1 wherein said fibril comprises multiple single crystals having multiple layers of graphene plane.
 8. The anode material of claim 1 which is prepared from a micron-diameter carbon fiber or graphite fiber having a diameter greater than or equal to 6 μm.
 9. The anode material of claim 1, wherein said submicron graphitic fibril is obtained by a method comprising (a) introducing an intercalating agent into inter-crystallite spaces and/or imperfections of a micron-diameter carbon fiber or graphite fiber to form an intercalated fiber; (b) activating said intercalating agent to split the intercalated fiber into an aggregate of multiple interconnected submicron graphitic fibrils having interconnections between fibrils; and (c) mechanically severing the interconnections to obtain the submicron graphitic fibril.
 10. The anode material of claim 9 wherein said intercalating agent further penetrates an inter-graphene plane space.
 11. The anode material of claim 9 wherein said step of introducing an intercalating agent is selected from the group consisting of chemical intercalating, electrochemical intercalating, gaseous phase intercalating, liquid phase intercalating, supercritical fluid intercalating, and combinations thereof.
 12. The anode material of claim 11 wherein said chemical intercalating comprises exposing said carbon fiber or graphite fiber to a chemical selected from the group consisting of sulfuric acid, sulfonic acid, nitric acid, a carboxylic acid, a metal chloride solution, a metal-halogen compound, halogen liquid or vapor, potassium permanganate, alkali nitrate, alkali perchlorate, an oxidizing agent, and combinations thereof.
 13. The anode material of claim 12 wherein said carboxylic acid is selected from the group consisting of aromatic carboxylic acid, aliphatic or cycloaliphatic carboxylic acid, straight chain or branched chain carboxylic acid, saturated and unsaturated monocarboxylic acids, dicarboxylic acids and polycarboxylic acids that have 1-10 carbon atoms, alkyl esters thereof, and combinations thereof.
 14. The anode material of claim 12 wherein said metal-halogen compound or halogen liquid or vapor comprises a molecule selected from bromine (Br₂), iodine (I₂), iodine chloride (ICl), iodine bromide (IBr), bromine chloride (BrCl), iodine pentafluoride (IF₅), bromine trifluoride (BrF₃), chlorine trifluoride (ClF₃), phosphorus trichloride (PCl₃), phosphorus tetrachloride (P₂Cl₄), phosphorus tribromide (PBr₃), phosphorus triiodide (PI₃), or a combination thereof.
 15. The anode material of claim 11 wherein said electrochemical intercalating comprises using a carboxylic acid as both an electrolyte and an intercalating agent source.
 16. The anode material of claim 11 wherein said electrochemical intercalating comprises imposing a current, at a current density in the range of 50 to 600 A/m², to said carbon fiber or graphite fiber which is used as an electrode material of an electrochemical apparatus.
 17. The anode material of claim 9 wherein said step of activating comprises exposing said intercalated fiber to a temperature in the range of 150° C. to 1,200° C.
 18. The anode material of claim 9 wherein said step of introducing an intercalating agent comprises using an acid as an intercalating agent and said step of activating comprises exposing said intercalated fiber to a temperature in the range of 600° C. to 1,100° C.
 19. The anode material of claim 1 wherein said fibril is prepared by using a method comprising (a) intercalating a carbon fiber or graphite fiber with an intercalating agent to form an intercalated fiber; and (b) thermally and/or chemically splitting or exfoliating said intercalated fiber to obtain said graphitic fibril.
 20. The anode material of claim 1 wherein said fibril is prepared by using a method comprising (a) intercalating a carbon fiber or graphite fiber with a supercritical fluid to form a tentatively intercalated fiber at a first temperature and a first pressure; and (b) exposing said tentatively intercalated fiber to a second temperature or a second pressure to obtain said graphitic fibril.
 21. The anode material of claim 1 wherein said fibril is coated with a thin layer of amorphous carbon after said splitting or separating step.
 22. A lithium ion battery containing the anode material of claim
 1. 23. A lithium ion battery anode material comprising a lower-micron graphitic fibril as an anode active material, wherein said fibril has a diameter or thickness equal to or greater than 1 μm but less than 6 μm and said fibril is obtained by splitting and separating a carbon fiber or graphite fiber of at least 6 μm in diameter.
 24. The anode material of claim 23 wherein said fibril diameter or thickness is in the range from 2 μm to 5 μm.
 25. The anode material of claim 23 wherein said fibril has a length greater than 10 μm.
 26. The anode material of claim 23 wherein said fibril is obtained by chemically or thermo-chemically splitting a carbon or graphite fiber into an aggregate of multiple interconnected or partially bonded fibrils and separating said graphitic fibril from said aggregate.
 27. The anode material of claim 23, wherein said graphitic fibril is obtained by a method comprising (a) introducing an intercalating agent into inter-crystallite spaces and/or imperfections of a micron-diameter carbon fiber or graphite fiber to form an intercalated fiber; (b) activating said intercalating agent to split the intercalated fiber into an aggregate of multiple interconnected submicron graphitic fibrils having interconnections between fibrils; and (c) mechanically severing the interconnections to obtain the graphitic fibril.
 28. The anode material of claim 27 wherein said intercalating agent further penetrates an inter-graphene plane space.
 29. The anode material of claim 27 wherein said step of introducing an intercalating agent is selected from the group consisting of chemical intercalating, electrochemical intercalating, gaseous phase intercalating, liquid phase intercalating, supercritical fluid intercalating, and combinations thereof.
 30. The anode material of claim 29 wherein said chemical intercalating comprises exposing said carbon fiber or graphite fiber to a chemical selected from the group consisting of sulfuric acid, sulfonic acid, nitric acid, a carboxylic acid, a metal chloride solution, a metal-halogen compound, halogen liquid or vapor, potassium permanganate, alkali nitrate, alkali perchlorate, an oxidizing agent, and combinations thereof.
 31. The anode material of claim 29 wherein said electrochemical intercalating comprises using a carboxylic acid as both an electrolyte and an intercalating agent source.
 32. The anode material of claim 27 wherein said step of activating comprises exposing said intercalated fiber to a temperature in the range of 150° C. to 1,200° C.
 33. The anode material of claim 27 wherein said step of introducing an intercalating agent comprises using an acid as an intercalating agent and said step of activating comprises exposing said intercalated fiber to a temperature in the range of 200° C. to 600° C.
 34. The anode material of claim 23 wherein said fibril is prepared by using a method comprising (a) intercalating a carbon fiber or graphite fiber with an intercalating agent to form an intercalated fiber; and (b) thermally and/or chemically splitting or exfoliating said intercalated fiber to obtain said graphitic fibril.
 35. The anode material of claim 23 wherein said fibril is prepared by using a method comprising (a) intercalating a carbon fiber or graphite fiber with a supercritical fluid to form a tentatively intercalated fiber at a first temperature and a first pressure; and (b) exposing said tentatively intercalated fiber to a second temperature or a second pressure to obtain said graphitic fibril.
 36. The anode material of claim 23 wherein said fibril is coated with a layer of amorphous carbon after said splitting or separating step.
 37. A lithium ion battery containing the anode material of claim
 23. 